Module 17
Base isolation
Period shift as a design strategy: what it buys in force, what it costs in displacement, how the isolated first mode should look, and the practical constraints — moat, services, wind restraint, re-centring.
What this module covers
- Derive the isolated period and explain why the superstructure's own stiffness barely enters
- Quantify the force–displacement trade using a response spectrum
- Recognise a healthy isolated first mode from its effective mass and period separation
- Size the isolator displacement and the moat that has to accommodate it
- State the practical constraints that decide whether isolation is viable at all
- Explain when isolation is the wrong answer
Lessons
- 17.1
The period shift
Why putting a soft layer under a building reduces the force it attracts, and exactly what that reduction costs.
Start lesson → The moat, the services, the wind restraint, the re-centring — and the sites and structures where isolation is the wrong answer.
Start lesson →
Module checkpoint
Check what you have taken in
4 questions
Question 1
What total isolator stiffness gives a 1 800 tonne building a period of 3.2 s? Answer in MN/m.
Question 2
A fixed-base period of 0.52 s becomes 3.2 s when isolated. What is the period shift ratio?
Question 3
An analysis gives a directional isolator displacement of 130 mm. Take the bidirectional resultant of two equal peaks, then apply torsional amplification 1.15 and a lower-bound bearing factor 1.20. What displacement results, in millimetres?
Question 4
For which of these should base isolation NOT be used?